1. Field of the Invention
The present invention relates to a technique for controlling temperature uniformity in during and in a crystal growth apparatus by utilizing a muffle.
2. Description of the Related Art
Crystal growth apparatuses or furnaces, such as directional solidification systems (DSS) and heat exchanger method (HEM) furnaces, involve the melting and controlled resolidification of a feedstock material, such as alumina or silicon, in a crucible to produce an ingot. Production of a solidified ingot from molten feedstock occurs in several identifiable steps over many hours. For example, to produce an ingot, such as a sapphire ingot, by the HEM method, solid feedstock, such as alumina, is provided in a crucible containing a monocrystalline seed (which comprises the same material as the feedstock but with a single crystal orientation throughout) placed into the hot zone of a solidification furnace. A heat exchanger, such as a helium-cooled heat exchanger, is positioned in thermal communication with the crucible bottom and with the monocrystalline seed. The feedstock is then heated to form a liquid feedstock melt, without substantially melting the monocrystalline seed, and heat is then removed from the melted feedstock by applying a temperature gradient in the hot zone in order to directionally solidify the melt from the un-melted seed. By controlling how the melt solidifies, a crystalline material having a crystal orientation corresponding to that of the monocrystalline seed, and having greater purity than the starting feedstock material, can be achieved.
Crystal growth typically requires highly uniform and controllable temperature gradients in order to insure the proper crystallization and growth. As such crystals, such as sapphire, are often produced in a vacuum furnace. In a vacuum, heat transfer is carried out by radiation, which in a vacuum is not as efficient as convection in a gaseous atmosphere or conductive heat transfer because there are fewer atoms to transport heat.
Accordingly, radiational heat transfer in a vacuum furnace is heavily affected by the emissivity of the heating element and crucible and the distance from the heating element to the crucible. In a vacuum with less than 0.1 torr, heat transfer is not as sensitive to small changes in vacuum levels as it is above a 0.1 torr vacuum level. Therefore, the decreased heat transfer in vacuum below 0.1 torr or 100 microns eliminates the need to tightly control the vacuum level below 0.1 torr.
However, vacuum levels below 0.1 torr result in non-uniform heat transfer from the heating element to a crucible where the crystalline material is being produced. Since in a vacuum there is low heat transfer, the temperature near the middle of the crucible is higher than near the top and bottom of the crucible. As such, the higher temperature nearer the crucible center than near the bottom of the crucible causes the crystal to grow in a dome shape due to the higher crucible wall temperature near the center of the crucible than near the bottom of the crucible. The higher temperature near the center of the crucible as a result causes hemispherical growth near the top of the crystal due to the higher temperature in the middle of the crucible and lower temperature near the bottom of the crucible. This results in domed shaped crystals near the center of the crucible and reduces the amount of material that can be utilized in product manufacture. Thus, controlling the shape of the boule that is output in the vacuum is exceedingly difficult.
Growing crystals in a vacuum environment may also cause decomposition of the melt. For sapphire growth, for example, this causes the top of some boules to vaporize and decompose into sub-oxides and O2 that are evacuated into the heat zone. Accordingly, the quantity of gases from the top surface of the melt depends on the diameter of the crucible, vapor pressure of the melt, vaporization of the crucible, and gaseous crucible melt interactions.
For example, experimental data has suggested that the light scatter in crystals increases as the crucible diameter increases since the surface area of the melt increases with the increase of the crucible diameter. Light scatter is understood by those skilled in the art as a measure of voids or inclusions in the crystal. In sapphire crystals, for example, there is almost no light scatter in eight-inch diameter sapphire crystals grown in vacuum. However, the light scatter does increase in sapphire crystals as the diameter of the melt surface is increased since the crystal's diameter increased. Light scatter in crystals is undesirable since it reduces the optical transmission particularly in ultraviolet (UV) light and above. Accordingly, the amount of light scatter is directly related to the crystal's diameter, and this reduces the amount of usable material from the crystal unless the diameter is properly controlled which is often dependent on temperature uniformity during crystal growth. One alternative to control temperature uniformity is helium. Helium is an excellent heat transfer gas used to achieve temperature uniformity by conductivity and convective heat transfer since helium gaseous materials are very small resulting in high convective heat transfer. However, the use of helium to achieve temperature uniformity has many disadvantages in the current systems. First, helium, a rare expensive gas, minimizes evaporation of impurities that decrease UV transmission, and causes a pink color throughout the crystal due to reactions with impurities that are not quickly flushed out of the hot zone as is the case for growth in vacuum. Also, growth in a helium atmosphere results in flat topped crystals, but with a pinkish color and lower ultraviolet impurities. Helium also requires more energy than vacuum growth. Thus, control under helium pressure is critical because changes in helium pressure affects heat loss, and therefore, pressure must be precisely controlled. Growth in a vacuum without helium thus preferred, however, there is currently no way to effectively control temperature uniformity and the shape of the boule in the vacuum without the use of such gaseous materials.